Page C-3
MPS Motor Protection System Rev. 6-F-022117
Appendix C, MPS Modbus Protocol
C.4.6 EXCEPTION RESPONSES
The MPS supports the following exception responses:
• Boundary Error (1)—Applies to writes of 32-bit
values. The high-order word must be written first
followed by the write to the low-order word. If this
sequence is not followed, a Boundary Error is
returned and the value will not stored. This does not
apply on read requests.
• Address Error (2)—All accesses to communication
registers must be within the specified address range
or the Address Error code is returned.
• Command Error (3)—This error code is returned if the
command code is not supported.
• Illegal Function Code (4)—The function code (Byte 2)
is not supported.
The exception message consists of the slave address
followed by a retransmission of the original function code.
The function code will have the most-significant bit set to
indicate an error. The 8-bit byte following the function code
is the exception response code. The 16-bit CRC is at the
end of the message.
C.5 MPS DATABASE
Appendix E contains the Modbus Register in the
Communications Database Table. The table starts at
register 0 (Modbus 40001) and each register is 16-bits wide.
Types “long” and “float” are 32-bit values. For both long
and float types, the low-order word is transmitted first
followed by the high-order word. Word values have the
high byte followed by the low byte. Float types as per IEEE
754 Floating-Point Standard. All bytes of long and float
types must be written using one message or an error will
result. This does not apply for read commands.
C.5.1 DATA RECORDS
Only one event record can be read at a time. Record data
is for the record indicated by the Record Selector. To select
a record, write the record number to Record Selector and
then read the values in the record. Record Head points to
the next available record. The last event record captured is
at Record Head minus one.
Both Record Selector and Record Head values are in the
range of 0 to 63. Values outside this range will select
record 0.
C.5.2 CUSTOM DATA ACCESS
Data access can be customized with the User-Defined
Registers and the User-Data Registers.
User-Defined Registers are located in non-volatile
memory and contain the register numbers from which data
is required. To access the data, read the corresponding
User-Data Registers.
The format of the User Data is a function of the
corresponding register entered in the User-Defined-Register
area.
C.6 NETWORK TIMEOUT
The MPS can be configured to trip or alarm on a network
timeout using the Setup | Hardware | Network Comms
menu. The Net Trip Action and Net Alarm Action set points
set the actions to be taken when a timeout occurs. To
prevent a timeout, a valid message, addressed to the slave,
must be received at time intervals less than five seconds.
NOTE: Set protocol to None before selecting Network Error
actions; then, select protocol.
C.7 SPECIFICATIONS
Interface ........................................ Isolated RS-485, 2-wire,
multi-drop, half duplex.
Protocol ........................................ Modbus RTU
Baud Rate ..................................... 1,200 to 19,200 bit/s
(3)
Bit Format .................................... 8 bits, no parity, one stop
bit
(1)
Number of CTU's Connected ...... Maximum of 32 units
Bus length ..................................... 1,200 m (4,000’) total
(2)
(1)
Terminal “-” is negative with respect to terminal “+” for
a binary 1 (MARK or OFF) state.
Terminal “-” is positive with respect to terminal “+” for
a binary 0 (SPACE or ON) state.
(2)
For line lengths exceeding 10 m (30’), 150-Ω terminations
are required at the cable ends.
(3)
Error rate is 1 byte/120 kB at 19,200 bit/s. For high
reliability, use 9600 bit/s.
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